In a typical refrigeration system, refrigerant circulates continuously through a closed circuit. The term "circuit", as used herein, refers to a physical apparatus whereas the term "cycle" as used herein refers to operation of a circuit, e.g., refrigerant cycles in a refrigeration circuit. The term "refrigerant", as used herein, refers to refrigerant in liquid, vapor and/or gas form. Components of the closed circuit cause the refrigerant to undergo temperature/pressure changes. The temperature/pressure changes of the refrigerant result in energy transfer. Typical components of a refrigeration system include, for example, compressors, condensers, evaporators, control valves, and connecting piping. Details with regard to some known refrigeration systems are set forth in Baumeister et al., Standard Handbook for Mechanical Engineers, McGraw Hill Book Company, Eighth Edition, 1979, beginning at page 19-6.
Energy efficiency is one of the important factors in the assessment of refrigeration systems. Particularly, an ideal refrigeration system operates at an ideal refrigeration effect. However in practice, an actual refrigeration system operates at less than the ideal refrigeration effect.
Increased energy efficiency is typically achieved by utilizing more expensive and more efficient refrigeration system components, adding extra insulation adjacent to the area to be refrigerated, or by other costly additions. Increasing the energy efficiency of a refrigeration system therefore usually results in an increase in the cost of the system. It is therefore, desirable to increase the efficiency of a refrigeration system and minimize any increase as a result thereof in the cost of the system.
In some apparatus utilizing refrigeration systems, more than one area needs to be refrigerated, and at least one area requires more refrigeration than another area. A typical household refrigerator, which includes a freezer compartment and a fresh food compartment, is one example of such an apparatus. The freezer compartment is preferably maintained between -10.degree. Fahrenheit (F.) and +15.degree. F., and the fresh food compartment is preferably maintained between +33.degree. F. and +47.degree. F.
To meet these temperature requirements, a typical refrigeration system includes a compressor coupled to an evaporator disposed within the household refrigerator. The terms "coupled" and "connected" are used herein interchangeably. When two components are coupled or connected, this means that the components are linked, directly or indirectly in some manner in refrigerant flow relationship, even though another component or components may be positioned between the coupled or connected components. For example, even though other components such as a pressure sensor or an expander are connected or coupled in the link between the compressor and evaporator, the compressor and evaporator are still coupled or connected.
Referring again to the refrigeration system for a typical household refrigerator, the evaporator is maintained at about -10.degree. F. (an actual range of about -30.degree. F. to 0.degree. F. is typically used) and air is blown across the coils of the evaporator. The flow of the evaporator-cooled air is controlled, for example, by barriers. A first portion of the evaporator-cooled air is directed to the freezer compartment and a second portion of the evaporator-cooled air is directed to the fresh food compartment. To cool a fresh food compartment, rather than utilizing evaporator-cooled air from an evaporator operating at about -10.degree. F., it is possible to utilize an evaporator operating at, for example, about +25.degree. F. (or a range of about +15.degree. F. to +32.degree. F.). A typical refrigeration system utilized in household refrigerators, therefore, produces its refrigeration effect by operating an evaporator at a temperature which is appropriate for the freezer compartment but lower than it needs to be for the fresh food compartment.
It is well-known that the energy required to maintain an evaporator at about -10.degree. F. is greater than the energy required to maintain an evaporator at about +25.degree. F. in a refrigerator. A typical household refrigerator therefore uses more energy to cool the fresh food compartment than is necessary, operating at reduced energy efficiency.
The above referenced household refrigerator example is provided for illustrative purposes only. Many apparatus other than household refrigerators utilize refrigeration systems which include an evaporator operating at a temperature below a temperature at which the evaporator actually needs to operate.
Refrigeration systems which operate at reduced energy consumption are described in commonly assigned U.S. Pat. Nos. 4,910,972 and 4,918,942. The patented systems utilize at least two evaporators and a plurality of compressors or a compressor having a plurality of stages. For example, in a dual, i.e., two, evaporator circuit for household refrigerators, a first evaporator operates at +25.degree. F. and a second evaporator operates at -10.degree. F. Air cooled by the first evaporator is utilized for the fresh food compartment and air cooled by the second evaporator is utilized for the freezer compartment. Utilizing the dual evaporator refrigeration system in a household refrigerator results in increased energy efficiency. Energy is conserved by operating the first evaporator at the temperature (e.g., +25.degree. F.) required for the fresh food compartment rather than operating an evaporator for the fresh food compartment at -10.degree. F. Other features of the patented systems also facilitate increased energy efficiencies.
To drive the plurality of evaporators in the refrigeration systems described in U.S. Pat. Nos. 4,910,972 and 4,918,942, and as mentioned above, a plurality of compressors or a compressor including a plurality of stages are utilized. Utilizing a plurality of compressors or utilizing a compressor having a plurality of stages results in increasing the cost of the refrigeration system over the cost, at least initially, of refrigeration systems utilizing one evaporator and one single stage compressor. It is therefore desirable to provide improved energy efficiency achieved by using a plurality of evaporators and to minimize, if not eliminate, the increase in cost associated with a plurality of compressors or a compressor having a plurality of stages.